Contradiction with John 14:31
In John's Gospel, there's no mention of singing a hymn before going out, but rather a discourse from Jesus, suggesting a different immediate sequence of events post-dinner.
John 14:31: But that the world may know that I love the Father; and as the Father gave me commandment, even so I do. Arise, let us go hence.
Contradiction with Matthew 26:36
While not directly contradictory, it expands on the narrative shortly after Matthew 26:30, so if reading interpretations strictly, there could be debates about what precisely transpired between the hymn and Gethsemane.
Matthew 26:36: Then cometh Jesus with them unto a place called Gethsemane, and saith unto the disciples, Sit ye here, while I go and pray yonder.
Contradiction with Mark 14:26
Similar to Matthew 26:30, mentions the hymn but focuses more on Jesus predicting Peter's denial immediately after, which could suggest thematic emphasis differences.
Mark 14:26: And when they had sung an hymn, they went out into the mount of Olives. [hymn: or, psalm]
Contradiction with Luke 22:39
While complementary rather than contradictory, omits the singing of the hymn and emphasizes discourse and going out directly to the Mount of Olives.
Luke 22:39: And he came out, and went, as he was wont, to the mount of Olives; and his disciples also followed him.